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The goal of this clinical research study is to find out if using the PR1 peptide vaccine (PR1) without PEG-Intron® (interferon) or in combination with interferon can reduce or eliminate disease in patients who have CML that is in cytogenetic remission after treatment with imatinib mesylate, but who still have small amounts of disease able to be noticed (detected). Researchers want to see if giving low doses of interferon together with PR1 may make the vaccine more effective. The safety of treatment in this study will also be studied.

Molecular response rate is number of respondents compared to total participants. Molecular Response is defined as a greater than a one-log reduction of Breakpoint Cluster Region-Abelson Murine Leukemia (BCR-ABL) transcript levels by quantitative polymerase chain reaction (PCR) from the baseline level at the time vaccination was initiated, or a disappearance of BCR-ABL transcripts, as measured by reverse transcription polymerase chain reaction (RT-PCR), occurring within 6 months from the last vaccination. Participants receive a series of 4 vaccinations administered at 3-week intervals and the fourth (final) vaccination administered 3 months after the third vaccination with blood draw to test PCR following every 3 months to test the level of leukemia in the blood and to see if disease is responding to the vaccine.

Secondary Outcome Measures:

Number of Participants With Immunologic Response [ Time Frame: Period of 18 weeks (i.e., one vaccination each on weeks 0, 3, 6, and 18). ]

Immunologic Response (immune response) is defined as an increase of ≥ 0.5 PR1-HLA-A2 [human leukocyte antigen-A2 (HLA-A2)] tetramer cells / μl at the time of either the 3rd or 4th vaccination compared to the pre study absolute PR1-HLA-A2 tetramer cells / μl. Participants receive a total of 4 vaccinations over a period of 18 weeks (i.e., one vaccination each on weeks 0, 3, 6, and 18). Participants assessed after 3rd and 4th vaccination for immunologic response.

PR1 peptide will be administered at a dose of 0.5 mg of PR1 on weeks 0, 3, 6 and 18 for a total of 4 doses. Continue receiving imatinib by mouth at the same dose received during the last 6 months. GM-CSF 75 micrograms subcutaneously in the same area as the vaccine with every vaccination.

Biological: Peptide Vaccine (PR1 Peptide)

PR1 peptide will be administered at a dose of 0.5 mg of PR1 on weeks 0, 3, 6 and 18 for a total of 4 doses.

Other Name: PR1 Vaccine

Drug: Imatinib

Continue receiving imatinib by mouth at the same dose received during the last 6 months.

Other Name: Gleevec

Drug: GM-CSF

75 micrograms subcutaneously in the same area as the vaccine with every vaccination.

Other Names:

Sargramostim

LeukineTM

Experimental: PR1 + Imatinib + Interferon

PR1 peptide will be administered at a dose of 0.5 mg of PR1 on weeks 0, 3, 6 and 18 for a total of 4 doses. Subcutaneous injection of interferon 0.5 microg/kg with each PR1 vaccination. GM-CSF 75 micrograms subcutaneously in the same area as the vaccine with every vaccination.

Biological: Peptide Vaccine (PR1 Peptide)

PR1 peptide will be administered at a dose of 0.5 mg of PR1 on weeks 0, 3, 6 and 18 for a total of 4 doses.

Other Name: PR1 Vaccine

Drug: Peginterferon alfa-2b

Subcutaneous injection of interferon 0.5 microg/kg with each PR1 vaccination.

Other Name: Peg-Intron

Drug: Imatinib

Continue receiving imatinib by mouth at the same dose received during the last 6 months.

Other Name: Gleevec

Drug: GM-CSF

75 micrograms subcutaneously in the same area as the vaccine with every vaccination.

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Patients must have received imatinib therapy for at least 18 months and not have increased their dose of imatinib in the last 6 months.

Patients must be in complete cytogenetic remission.

Patients must have detectable BCR-ABL transcript levels meeting at least one of the following criteria: 1) Patient has never achieved a major molecular response (i.e., never reached levels <0.05%), and transcript levels have shown in at least two consecutive measures separated by at least 1 month to have increased by any value, or

continued from above: 2) Achieved a major molecular response that has been lost with an increase in transcript levels by at least 1-log over two consecutive analyses separated by at least 1 month, or 3) BCR-ABL transcript levels have reached a plateau defined as a ratio that is not more than 0.25-log (one fourth of a log) lower than the lowest value obtained in the last at least 6 months, with at least 2 values obtained during this period.

Patients must not have had a continuous interruption of imatinib therapy of greater than 14 days or any interruptions totaling 6 weeks within the 6 months prior to enrollment.

Patients must be HLA-A2 positive at one allele

Patients must give informed consent and sign an informed consent indicating that they are aware of the investigational nature of this study in keeping with the policies of the hospital.

Chronic use (> 2 weeks) of greater than physiologic doses of corticosteroid agent (dose equivalent to > 10 mg/day of prednisone) within 28 days of the first day of study drug treatment (topical and inhaled corticosteroids are permitted)

GM-CSF or interferon administration within 1 month of first PR1 injection

Patients receiving any other investigational agents currently or within the past 4 weeks. Patients must have recovered from any adverse effects of investigational therapy.